possible, yes.
but I'm unclear as to what you mean. as i understood everything was staying the same save that you were upgrading your WP installation from 2.8.x to 3.0.x.
there is a database upgrade between the two, done automatically. which may make it difficult to roll back. worth taking a db backup first.
all core features will work, however plugins that you used on the old installation may not work on the new. you should check compat on the plugin authors site and on wordpress.org/plugins
feel free to post more detailed questions in the wordpress forum